Why do coral polyp tentacles open up at night time and stay closed during day. why the night and day?

Question

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Answer 1

While corals do feed at night, not all stay closed during the day.  Many also feed in daylight as well.

However, the reason why there is more feeding activity during the night is because corals have zooxanthellae, which are endosymbiotic dinoflagellates that photosynthesize during the day, providing sugars to the coral polyp.  At night, the zooxanthellae are not photosynthetically active and therefore must feed.  

Additionally, the food source for corals (primarily zooplankton) vertically migrate and come up into shallower waters during the night.  So, the food source for corals is more abundant at night than during the day.